1. Field of the Invention
The invention relates in general to an ESD protection circuit, and more particularly to a substrate-triggered ESD protection circuit.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional substrate-triggered ESD protection circuit. As shown in FIG. 1, for an input pad 110, it will receive an external signal S to an internal circuit 120, such as an input buffer. An ESD protection circuit 100 is connected between the input pad 110 and internal circuit 120 for protecting the internal circuit 120 from ESD damage. The ESD protection circuit 100 includes a primary device 102, a resistor R, and a secondary device 104. The resistor R is connected between the input pad 110 and internal circuit 120. The primary device 102 is connected to the input pad 110 and one end of the resistor R for clamping the voltage of the input pad 110, and the secondary device 104 is connected to the internal circuit 120 and the other end of the resistor R for clamping the voltage of the internal circuit 120.
The primary device 102 further includes a substrate pump 102a and a main ESD protection device 102b, such as a field transistor with a parasitic bipolar junction transistor (BJT). The main ESD protection device 102b is connected to the input pad 110 and resistor R. The substrate pump 102a is connected to the input pad 110 and the main ESD protection device 102b for pumping the substrate voltage of the main ESD protection device 102b. 
When an ESD current enters the input pad 110, the voltage of the input pad 110 will rise rapidly and the secondary device 104 is turned on first to clamp the voltage of the internal circuit 120. The voltage of the input pad 110 keeps rising until the substrate pump 102a is turned on to drain the ESD current into the bulk of the main ESD protection device 102b to raise its potential. Higher bulk potential can speed up the turn-on of the main ESD protection device 102b and make it drain most of the ESD current more efficiently.
However, the substrate pump 102a is conventionally formed by a RC circuit or a RC-coupled inverter. In order to couple enough voltages from the input pad 110 to the bulk of the main ESD protection device 102b, a large area of the RC circuit or RC-coupled inverter needs to be used, which increases the production cost. In some applications, the substrate pump 102a is implemented by a ground-gate N-type metal oxide semiconductor (NMOS) transistor. Because of the gate-aided effect, the ground-gate NMOS transistor turns on before the main ESD protection device 102b and may fail first if the protection device 102b cannot turn on immediately, therefore the substrate pump 102a will be damaged.